An optical instrument has an optical response that varies as a function of its azimuth and elevation viewing angles from a centered coordinate system. As such, an important characteristic of the instrument is the optical response of the instrument within its field-of-view (FOV). Although two instruments may be built to the same specification, each instrument may have a different optical response. Each instrument, therefore, is individually tested to measure optical response as a function of its FOV, or, stated differently, tested to measure its FOV.
There are many methods of measuring an instrument's FOV. Most is methods require setting up a grid of spot measurements. A grid of spot measurements requires a two-dimensional grid of evenly spaced points, positioned in an area intercepting the FOV of the instrument. Collimated beams are sent into the instrument, at positions corresponding to the spots on the grid. The response of the instrument to each beam is recorded, forming a set of information describing the instrument's FOV.
For many types of optical instruments, setting up a grid of spot measurements is adequate. Although the setup may require a large number of grid points and a large number of collimated beams to measure the FOV, the task may be done in a reasonable amount of time provided that each grid point can be measured quickly. Since the quality of the measurement depends on the number of photons, or the amount of light, in the collimated beam, the setup must include collimated beams that are strong enough to permit each grid point to be measured quickly.
Providing strong collimated beams for each grid point is not practical, when measuring the FOV of a spectrometer, for example. It is desirable for the collimated beams to exhibit a specified spectrum and include relatively weak collimated beams at some wavelength of the frequency spectrum. These weak collimated beams require a relatively long period of time to collect a sufficient amount of photons. Thus, the FOV measurement also requires a long period of time.
A need exists for a method of measuring a FOV of an optical instrument, device, or sensor, that may be performed in a much shorter period of time than those required by conventional measurement methods. The present invention addresses such need.